This invention relates to the mitigation of echoes in a communication circuit and, more particularly, to the cancellation of echoes in a two-way communication circuit.
Echoes commonly occur in communication systems when electrical signals on a first, incoming transmission path meet an imperfectly matched impedance at a hybrid junction and are partially reflected back to the source over a second, outgoing transmission path. Typically, signals such as speech or data signals require a nonzero travel time. As a result, the reflected signal, or echo, is heard at the far end of the second path some time after the speech signal is transmitted from the near end of the second path. As the distance between the talking and listening parties is increased, the echo takes longer to reach the talking party. As a result, the echo becomes, at least qualitatively, more annoying to the talking party. An attempt is therefore generally made to control echoes. One echo controlling arrangement includes a speech signal operated device known as an echo suppressor. Conventional echo suppressors combat echoes by interrupting signals on the second outgoing transmission path according to some decision based upon the relative levels of the incoming and outgoing signals. On the other hand, rather than interrupt the outgoing path, another echo controlling arrangement, known as an echo canceller, typically synthesizes a signal estimate of the echo signal and algebraically subtracts the estimate from the outgoing signal to obtain an error signal. Inasmuch as the echo is included in the outgoing signal, the subtraction effectively cancels the echo. For example, J. L. Kelly, Jr. and B. F. Logan in U.S. Pat. No. 3,500,000 issued Mar. 10, 1970 discloses such an echo canceller. That canceller includes an adjustable signal processor having a closed loop error control system which is self-adapting in that it automatically tracks signal variations in the outgoing path. More particularly, that echo canceller employs estimator apparatus including a transversal filter arrangement for synthesizing a linear approximation of the echo. The filter comprises a delay line having a number of taps, say an integer N taps, spaced along its length at convenient Nyquist intervals. Thereby, the echo canceller develops a number of delayed replicas of the incoming signal. The gain and polarity of each replica is independently adjusted in response to the error signal, and hence in response to the echo detected in the outgoing path. The adjusted signals are then algebraically combined to obtain the synthesized echo estimate for subtraction from the outgoing signal. Thereby, the echo is cancelled.
An improvement thereover is disclosed by M. M. Sondhi in U.S. Pat. No. 3,499,999, also issued Mar. 10, 1970. In Sondhi, an adaptive echo canceller includes a speech detector circuitwise situated to receive at a first input a signal incoming from the far end, which incoming signal is also extended to one input of a hybrid, and at a second input a near end signal outgoing to the far end, which outgoing signal is extended from an output of the hybrid. A control signal output of the speech detector is supplied to a control input of a control switching gate for closing or opening the gate. Usually, the error signal is provided to a first input of the gate for extension through the closed gate to the estimator apparatus. On the other hand, the gate is typically opened upon detection of near end speech, for example, during a situation in which both near end and far end parties are simultaneously talking. Responsive to the opened gate, the echo signal estimate is inhibited from being adjusted. To understand an advantage of the inhibiting, the outgoing signal may include not only an echo signal but also a near end speech signal. As a result, it is usual to account for the near end speech signal during the adjustment. For example, in the Sondhi canceller, on the one hand, further adjustment of the estimate is inhibited upon detection of the presence of near end speech. On the other hand, absent near end speech, the adjustment may occur. Hence, during the adjustment, the error signal is substantially the difference between the true echo signal and the estimate thereof. Thus, in accounting for the near end signal, the signal processor can be designed to adjust the estimate to converge to a zero value error signal. After the estimate is adjusted and the error signal does converge, subsequent occurences of a large error signal are typically indicative of the presence of a near end speech signal. Unfortunately, if a large error signal is taken as an indication of a near end speech signal, known echo cancellers may not converge since the estimator apparatus may inhibit further adjustment of the estimate and the error signal may remain large.
Accordingly, an object of the present invention is to provide an improved echo canceller for mitigating an echo signal in the presence of a relatively wide range of echo signals.